1. Field of the Invention
This invention relates in general to an apparatus for controlling gas flow for combustion in a furnace. More particularly, the invention relates to an improved valve for adjusting the gas flow in a furnace in response to downstream pressure changes.
2. Discussion of the Related Art
In conventional gas-fired forced air furnaces a thermostat senses the temperature in the comfort zone relative to a predetermined set point temperature. When the temperature is below the set point, the thermostat closes to supply thermostat ac power to the furnace as a call for heat. This initiates a sequence of events that ultimately causes the furnace to come on. A draft inducer motor is enabled to flow air through the heat exchangers for combustion, after which a gas valve is actuated to supply gas to the gas burners. An ignition device is also actuated to light the burners. A flame sensor then proves burner ignition. Then, after a predetermined blower delay time, which varies with furnace design, the furnace blower is actuated. The blower circulates room air from the return air duct over the furnace heat exchangers to pick up heat from the hot combustion products (carbon dioxide and water vapor). The heated circulating air then goes into the supply air plenum and is distributed by ductwork back to the living space. When the living space is warmed sufficiently to reach the thermostat set point, the thermostat terminates the call for heat. When this happens, the blower and burners go through a shut off sequence and the furnace awaits the next call for heat.
The present invention relates to the control of gas flow to the burners. When the draft inducer motor is in operation, a substantial step-up in pressure occurs between the intake of the draft inducer housing (the collector box) on the one hand, and the outflow of the draft inducer housing (the relief box) on the other hand. Typically there is negative pressure (relative to atmospheric pressure) at the intake, and positive pressure at the outlet.
The negative pressure is used to draw combustion air through the furnace heat exchangers. The positive pressure results when the furnace is installed as a category III vented appliance. Under certain outside conditions, such as high wind conditions, back pressure on the vent causes the draft inducer to become overloaded. The overloading of the draft inducer prohibits the device from providing the air required for proper combustion. Operating under this lean condition, the furnace can produce unwanted products of combustion, such as carbon monoxide. Therefore, an apparatus is needed which senses the pressure changes caused by changing wind conditions and adjusts the gas flow to the burners accordingly.